This invention relates to an asynchronous connecting device for a communications system in which an analog signal is transmitted after being converted into a digital signal.
A digital transmission system is superior to an analog transmission system in that the former is more economical and provides better signal transmission quality than the latter. As semiconductor techniques, such as LSI techniques, and digital technology have advanced, it has become increasingly common to transmit a digital signal instead of an analog signal. This tendency is significant, since a variety of codecs (encoder-decoders) for pulse code modulation are commercially available.
When it is necessary to transmit an analog signal, such as a voice signal, in a digital mode, the analog signal is first pulse-code -modulated by an encoder. The resultant PCM signal is supplied, as a digital signal of 64 kbps (kilo-bits/sec) for instance, to the transmission system. The transmission systems can be classified into a first group of transmission systems in which transmission paths are connected to one another in a synchronous manner, and a second group of transmission systems in which transmission paths are connected to one another in an asynchronous manner.
FIG. 1 shows the essential parts of a communications system according to the first type. The analog output of a transmitter 11 is applied to an encoder 12 where it is sampled and quantized, and is then encoded according to the obtained amplitude, to provide a digital signal. The digital signal thus provided is transmitted over a digital transmission path 13. For instance, a transmission clock signal and a byte clock signal are transmitted, as a PCM signal, through the digital transmission path. The digital signal, being synchronized, is decoded into an analog signal by a decoder 14. The analog signal thus obtained is supplied to a receiver 15 for reproducing the voice data.
FIG. 2 shows the essential parts of a communications system according to the second type. In this communications system, two digital transmission paths 21 and 22 are asynchronously connected to each other. In this case, the digital signal which is transmitted from one digital transmission path 21 is decoded into an analog signal by a decoder 23, and the analog signal is then encoded by an encoder 24. This communications system suffers from the following difficulties:
(1) A decoding and coding operation must be carried out at the connecting point of the transmission paths, and the manufacturing cost of the communications system is increased accordingly; and
(2) The S/N ratio is decreased, since the quantization distortions or noises, which are formed when the analog signal is sampled and quantized, are accumulated to thereby increase the noise.
If the decoder 23 and the encoder 24 are eliminated and the transmission path 21 is connected directly to the transmission path 22, then the serial data bits and the transmission clock signals may overlap or be partially lost. Therefore, in the decoder 14 on the signal receiving side, the byte clock signals will not be synchronized, so that the analog signal, e.g. a voice signal, may be reproduced as a completely different signal. Thus, asynchronously connectng the transmission paths without using a codec lowers the transmission quality, and it is therefore difficult from a practical standpoint to use such communications systems.